36.7: Responses to Salt Stress
Salt stress—which can be triggered by high salt concentrations in a plant’s environment—can significantly affect plant growth and crop production by influencing photosynthesis and the absorption of water and nutrients.
Plant cell cytoplasm has a high solute concentration, which causes water to flow from the soil into the plant due to osmosis. However, excess salt in the surrounding soil increases the soil solute concentration, reducing the plant’s ability to take up water.
High levels of sodium are toxic to plants, so increasing their sodium content to compensate is not a viable option. However, many plants can respond to moderate salt stress by increasing internal levels of solutes that are well-tolerated at high concentrations—like proline and glycine. The resulting increased solute concentration within the cell cytoplasm allows the roots to increase water uptake from the soil without taking in toxic levels of sodium.
Sodium is not essential for most plants, and excess sodium affects the absorption of essential nutrients. For example, the uptake of potassium—which regulates photosynthesis, protein synthesis, and other essential plant functions—is impeded by sodium in highly saline conditions. Calcium can ameliorate some effects of salt stress by facilitating potassium uptake through the regulation of ion transporters.
Not all plants are sensitive to salt. Plants can be classified as halophytes or glycophytes based on their salinity tolerance. While halophytes are salt-resilient, glycophytes are not. In order to tolerate high salt concentrations, halophytes may reduce sodium uptake, compartmentalize sodium, or excrete sodium. A small group of halophytes called recretohalophytes have specialized epidermal glands—called salt glands—in their stems and leaves. Salt glands take up excess salt from neighboring tissues and excrete it onto the plant surface. By studying halophytes, scientists can uncover the mechanisms of salt tolerance in plants and potentially use this knowledge to improve crop production in regions affected by salinity.